The axion [1]} is the quasi-Nambu-Golstone boson
resulting from the spontaneous breaking of the U\(_{\rm PQ}(1)\) 
anomalous global symmetry that Peccei and Quinn [2]}
postulated. The properties of the axion depend for the
most part on a single parameter \(f_a\)  called the axion
decay constant. In particular the axion mass is
\(m \simeq 6~\mu {\rm eV}\left({10^{12}~{\rm GeV} \over f_a}\right)\) 
